The invention relates to a method of continuous casting of ingots, which is realized in radial continuous casting plants.
In the past ten years, radial and especially curvilinear plants of continuous casting have been extensively gaining acceptance since they are fit for casting large-size ingots with a cross-sectional area amounting to 300.times.2000 mm. The speed of the ingot drawing (extraction) from the cooled mould reaches 2 m/min.
Large-size ingots cannot be practically cast at such speeds in the known vertical plants used so far.
In radial plants, the ingot is straightened right after it has passed the downcast portion.
However, in the course of ingot straightening, if the ingot has not yet solidified, marked strectching deformations emerge on the solidification boundary, that may cause inner defects in the ingot. To deal with them, a cooled mould with a large curvature radius was provided, that lead, however, to an increased height of the plant.
In the curvilinear plant, the ingot is being straightened gradually within a secondary cooling zone section of a considerable length; therefore, stretching deformations on the metal solidification boundary are negligible and do not cause dangerous deformations. An increase in the height of the casting plant can be thus avoided.
Known in the art is a method of producing massive metal ingots in a radial continuous casting plant intended for producing hollow pieces, according to which, first to be formed is a downcast portion of the ingot, with an upcast portion being formed second (Austrian Pat. No. 286519). Both the downcast and upcast portions of the ingot form a solidified envelope enclosing molten metal, a hollow portion being formed in the upcast branch above the level of the molten metal. The upper hollow portion of the ingot is exposed to a sizeable reduction right in the plant, which ensures both the sealing of the hollow of the ingot envelope and further deformation (shaping) of an ingot portion with a continuous cross section.
The reduction of the ingot hollow envelope by this method takes place above the level of the molten metal therein in the upcast branch. Formed between the metal meniscus in the upcast branch and the reduced portion of the ingot is a closed volume termed a "gas cushion".
According to the technique described in the Austrian Patent, the "gas cushion" in the ingot envelope is created after the ingot has passed the downcast portion prior to passing the upcast one. The "gas cushion" produces no effect upon the initial formation of the skin (envelope) of the ingot; neither does it influence the formation of the structure of the solidifying skin or assist the formation of the multilayer ingot.
Also, quite probable variations in the physical parameters of the "gas cushion" such as temperature, volume, and pressure may cause fluctuations in the level of the meniscus of the molten metal in the mould, which is undesirable in the course of the ingot's downward travel out of the mould.
Even short pauses in the travel of the ingot being cast may result in the solidification of the metal meniscus within the gas cushion zone, which may cause the compression of the gas cushion with the subsequent destruction of the solidified meniscus and the discharge of the molten metal in the zone of the mould being cooled.
Also, the completion of the ingot casting is impeded as it is not possible to continue the reduction of the hollow ingot after its end has escaped from the mould being cooled because otherwise this would result in the discharge of the liquid phase of the ingot into the secondary cooling zone. To prevent this, it is necessary to ensure complete crystallization of the downcast and upcast portions of the ingot being cast, but this would require more powerful facilities for reducing such ingot to a present section.
Known also in the art is a method of continuous casting of ingots from liquid metal fed from a vessel through a bottom gate to a curvilinear mould cooled radially, with a dummy bar being introduced into the space of the mould prior to casting. In forming an upcast radial branch of the ingot, the latter is regularly withdrawn upward from the mould into a secondary cooling zone. During the ingot formation, the pressure of the molten metal in the envelope of the ingot being cast is raised by cycles in the mould zone at the beginning of the pause between ingot drawings and then reduced at the end of the pause (USSR Inventor's Certificate No. 265,385. The rise in the molten metal pressure on the ingot skin ensures its fitting along the walls throughout the mould height.
Prior to drawing the solidified skin of the ingot from the mould, the pressure in the bottom gate should be reduced, and the molten metal in the ingot being cast is balanced with the aid of an electromagnetic field induced by inductors arranged in the upcast branch of the zone of the secondary cooling device.
As a result, the solidifying skin of the ingot in the mould zone is released from the metallostatic pressure and its further drawing from the mould is facilitated, as this drawing can be performed without causing the destruction of the ingot skin.
The continuous casting of the ingot with its periodic upward drawing from the radial cooled mould, as described in the above Inventor's Certificate, offers considerable advantages over the known method of continuous casting through the radial mould with the downward ingot drawing, as it helps increase the average speed of the ingot travel and, consequently, the efficiency of the plant, as well as improve the surface finishing of the ingot being cast.
However, in realizing this method for casting a number of metals, e.g., steel, a mould with a length of more than 1.2-1.4 m cannot be used, as it does not help reduce the load on the ingot skin to a required value prior to drawing the ingot from the mould. Meanwhile, in the course of the periodic drawing of the ingot from the mould, the average speed of the ingot travel and, consequently, the efficiency of the plant is nearly directly proportional to the length of the mould, and the greater its length, the higher the average speed of the ingot travel and the efficiency of the plant.
Furthermore, a limited length of the mould leads to a greater number of joints in the ingot cast continuously, which is undesirable.
In producing large-size ingots, the liquid phase in the ingot being cast spreads throughout the upcast portion. For its imperative balancing prior to the ingot drawing from the mould, a relatively strong magnetic field is required, which, necessitates, consequently, the provision of powerful inductors that cannot but complicate the construction of the plant.
This method does not permit the casting of multilayer ingots, whereas the production of profiled ingots through such techniques is impeded.
Note should be made that despite a variety of methods and plants developed in the sphere of continuous casting of metals, a number of problems still await their solution.
With the efficiency of modern metallurgical works being on the rise, present-day plants yet fail to provide adequate casting of all of a metal into ingots of a required section within a prescribed time. Therefore, they have to resort sometimes to an unwarranted increase in the size of ingots being cast and, naturally, the subsequent rolling of the latter necessitates the provision of the related plants of a greater capacity. To cope with the problem, it is necessary to provide for the techniques which permit casting at a far greater average speed of travel of the ingot being cast.
Another important problem of continuous casting, not solved completely, is to raise the quality of the ingot being cast (better surface finishing and improved inner structure).
Present-day requirements call for more bimetallic or multilayer ingots whose production process still remains complicated and expensive; in fact, there are no effective methods that help cast massive multilayer ingots continously.
Known is a method of producing bimetallic or multilayer ingots of limited lenths, involving centrifugal casting with a gradual introduction of varying chemical compounds into the metal form.
Another method involves the production of multilayer ingots through rolling packets of sheets made up of heterogeneous metals and prepared in advance.
In the method described in the U.S. Pat. No. 3,625,277, when casting metal for the production of multilayer ingots, the mould is moved toward the ingot being cast, i.e., horizontally. As this takes place, metal of a different chemical composition, intended for forming the ingot layers, is fed in turn to the space of the solidified skin of the ingot.
Bimetallic ingots cast continuously are produced by applying the second layer of the molten metal on the base prepared of the first metal in advance.
However, the first three methods of producing multilayer ingots involve the recurrent casting of metal and permit casting of ingots of limited lengths.
Also, prior to producing a multilayer ingot by the above-specified methods, it is necessary to do laborious preparatory operations such as, for instance, surface planning, degreasing, removing oxidized surface films chemically, and preventing the metals to be joined from repeated oxidation.
In producing the multilayer ingot of a limited length, too, in the method described in the U.S. Pat., it is practically impossible to produce an ingot with a clear-out boundary between the metal of one chemical composition and the layer of a metal of another chemical composition, because the second metal is introduced into the ingot space when there is a liquid phase of the first metal therein.
Moreover, each of the obtained layers may be of a different thickness throughout the ingot length.
The known methods of continuous casting of multilayer ingots rail to ensure the production of quality ingots as they do not create necessary conditions for the diffusion of one metal into another.